Thin SiO2 films, a process for producing them and their use

ABSTRACT

Thin SiO 2  films can be produced by hydrolysis and condensation of 
     a) 40 to 100% by weight of one or more silanes of general formula (I) 
     
         R.sub.x --Si--A.sub.4-x                                    (I) 
    
      in which the groups A are identical or different and stand for hydroxyl groups or hydrolytically separable groups, the groups R are identical or different and stand for hydrolytically non-separable groups, x has the value 0, 1, 2 or 3, x being not less than 1 for 70% by moles of said silanes; 
     b) optionally in the presence of 0 to 50% by weight of colloidal SiO 2  and/or 
     c) 0 to 10% by weight of organic binder. 
     The viscous sol thus obtained is worked into a gel film which is heat-treated.

The present invention relates to thin, tearfree, preferably transparentand colorless SiO₂ films, a process for producing them according to thesol-gel process and their use as, e.g., membranes, filters, componentsof laminates or support materials having functional additivesincorporated therein.

Films made of glass are usually produced by drawing or extrusionprocesses from the melt. Said processes depend on the thermal propertiesof the glass (softening point, crystallization rate etc.) and thuslimited to specific glass compositions. There is a further limitationregarding the minimum thickness of the glass films which can beproduced, this being the reason that so far it has not been possible toproduce films of silica glass having a thickness below about 250 μm bymelt and subsequent molding processes.

It is known that by sol-gel techniques the densification temperature ofvitreous and/or ceramic materials can be substantially reduced. However,the production of SiO₂ films according to the sol-gel process has so farbeen prevented by the problems in the conversion of the mostlyaqueous-alcoholic precursors (sols) to xerogel bodies by solvent removalor addition of a gelling agent. In the drying process of sols onsubstrates the formation of tears can easily take place due to capillaryforces and according to different drying rates at the upper and lowerside, respectively. Furthermore, due to the preferential solventevaporation at the surface in the course of the drying operation saidgel films separate from the substrate and roll up. A process accordingto which transparent, tearfree SiO₂ films having larger dimensions canbe produced by casting and gelling of SiO₂ sols on a base has not beenknown.

Surprisingly it has now been found that thin SiO₂ films of substantiallyany dimensions (length and width) can very easily be produced in asol-gel process.

Object of the present invention is a process for the production of thinSiO₂ films which is characterized in that

a) 40 to 100% by weight of one or more silanes of general formula (I)

    R.sub.x --Si--A.sub.4-x                                    (I)

wherein the radicals A are the same or different and represent hydroxylgroups or hydrolytically removable groups, the radicals R are the sameor different and are hydrolytically non-removable groups and xrepresents 0, 1, 2 or 3, provided that x≧1 in at least 70% by moles ofsaid silanes; optionally in the presence of

b) 0 to 50, preferably 0 to 25% by weight of colloidal SiO₂ and/or

c) 0 to 10% by weight of an organic binder

are hydrolyzed and condensed, the resulting viscous sol is worked into agel film and said gel film is heat-treated.

Objects of the present invention also are SiO₂ films which can beproduced in said manner and SiO₂ films having a thickness of 2.5 to 250μm and a surface area of at least 25 cm², preferably at least 50 cm²,particularly preferred at least 100 cm².

The SiO₂ films according to the present invention are thin, tearfree,preferably transparent and colorless, and are characterized by a highflexibility and minimum shrinkage.

Details regarding the sol-gel process are disclosed in C. J. Brinker, G.W. Scherer: "Sol-Gel Science--The Physics and Chemistry ofSol-Gel-Processing", Academic Press, Boston, San Diego, New York, Sidney(1990), and in DE 1941191, DE 3719339, DE 4020316 and DE 4217432.

Said references also describe specific examples of the silanes which canbe employed in the process according to the present invention as well asof hydrolytically removable radicals A and hydrolytically non-removableradicals R.

Preferred examples of hydrolytically removable groups A are hydrogen,halogen (F, Cl, Br and I, particularly Cl and Br), alkoxy (particuarlyC₁₋₄ alkoxy such as methoxy, ethoxy, n-propoxy, i-propoxy and butoxy),aryloxy (particularly C₆₋₁₀ aryloxy, e.g. phenoxy), alkaryloxy (e.g.benzyloxy), acyloxy (particularly C₁₋₄ acyloxy such as acetoxy andpropionyloxy) and alkylcarbonyl (e.g. acetyl). Radicals A which are alsouseful include amine groups (e.g. mono- or dialkyl, aryl and aralkylamino groups featuring the above mentioned alkyl, aryl and aralkylradicals), amide groups (e.g. benzamido) and aldoxime or ketoximegroups. Furthermore, two or three radicals A together may form agrouping complexing the Si atom, this being the case e.g. with Si-polyolcomplexes derived from glycol, glycerol or brenzcatechol.

The above hydrolyzable groups A may optionally have one or more usualsubstituents, e.g., halogen atoms or alkoxy groups.

The hydrolytically non-removable radicals R are preferably selected fromalkyl (particularly C₁₋₄ alkyl such as methyl, ethyl, propyl and butyl),alkenyl (particularly C₂₋₄ alkenyl, such as vinyl, 1-propenyl,2-propenyl and butenyl), alkynyl (particularly C₂₋₄ alkynyl such asacetylenyl and propargyl), aryl (particularly C₆₋₁₀ aryl such as phenyland naphthyl) and the corresponding alkaryl and arylalkyl groups.Optionally, these groups may also feature one or more usual substituentssuch as halogen, alkoxy or epoxide groups.

The above mentioned alkyl, alkenyl and alkynyl groups include thecorresponding cyclic radicals, such as e.g. cyclohexyl.

The silanes of general formula (I) employed according to the presentinvention may be employed, completely or in part, in the form ofprecondensates, i.e., compounds which have been formed by partialhydrolysis or the silanes of formula (I), either alone or in mixturewith other hydrolyzable compounds. Such oligomers which are preferablysoluble in the reaction medium may be straight-chain or cyclic lowmolecular weight partial condensates (polyorganosiloxanes) having adegree of condensation of e.g. about 2 to 100, particularly about 2 to6.

Specific examples of silanes of general formula (I) are compounds of thefollowing formulae:

Si(OCH₃)₄, Si(OC₂ H₅)₄, Si(O-n- or i-C₃ H₇)₄, Si(OC₄ H₉)₄, SiCl₄,HSiCl₃, Si(OOCCH₃)₄, CH₃ --SiCl₃, CH₃ --Si(OC₂ H₅)₃, C₂ H₅ --SiCl₃, C₂H₅ --Si(OC₂ H₅)₃, C₃ H₇ --Si(OCH₃)₃, C₆ H₅ --Si(OCH₃)₃, C₆ H₅ --Si(OC₂H₅)₃, (CH₃ O)₃ --Si--C₃ H₆ --Cl, (CH₃)₂ SiCl₂, (CH₃)₂ Si(OCH₃)₂, (CH₃)₂Si(OC₂ H₅)₂, (CH₃)₂ Si(OH)₂, (C₆ H₅)₂ SiCl₂, (C₆ H₅)₂ Si(OCH₃)₂, (C₆H₅)₂ Si(OC₂ H₅)₂, (i-C₃ H₇)₃ SiOH, CH₂ ═CH--Si(OOCCH₃)₃, CH₂ ═CH--SiCl₃,CH₂ ═CH--Si(OCH₃)₃, CH₂ ═CH--Si(OC₂ H₅)₃, CH₂ ═CH--Si(OC₂ H₄ OCH₃)₃, CH₂═CH--CH₂ --Si(OCH₃)₃, CH₂ ═CH--CH₂ --Si(OC₂ H₅)₃, CH₂ ═CH--CH₂--Si(OOCCH₃)₃, CH₂ ═C(CH₃)--COO--C₃ H₇ --Si(OCH₃)₃, CH₂ ═C(CH₃)--COO--C₃H₇ --Si(OC₂ H₅)₃, ##STR1##

Said silanes may be prepared according to known methods; cf. W. Noll,"Chemie und Technologie der Silicone", Verlag Chemie GmbH,Weinheim/Bergstraβe (1968).

The SiO₂ films according to the present invention may be prepared, e.g.,from methyltriethoxysilane (MTEOS) alone or from mixtures of MTEOS andtetraethoxysilane (TEOS). A particularly preferred system comprises 90%by moles of MTEOS and 10% by moles of TEOS.

In order to improve the handlability and other mechanical properties ofthe thin SiO₂ films colloidal SiO₂ is preferably employed as additionalstarting material. The use thereof also increases the density and thepore volume of the films. Said colloidal SiO₂ may be present, e.g., inthe form of silica sols or nanoscaled dispersible powders. An alcoholicsilica sol having a particle size of about 10 nm is preferred.

As further optional component an organic binder which can subsequentlybe removed again from the gel film may be employed. Suitable bindersare, e.g., polyvinyl alcohol, polyvinyl acetate, starch, polyethyleneglycol and gum arabic.

The hydrolysis and condensation is carried out under sol-gel conditions,preferably in an alcoholic solvent (e.g. methanol or ethanol) with basiccatalysis (e.g., using ammonia) until a viscous sol has formed. In orderto adjust a favourable morphology of the sol particles and viscosity ofthe sol the hydrolyzed/condensed product is preferably subjected to anaging step wherein the reaction mixture is heated to temperatures offrom 40 to 120° C. for several hours to several days. Heating to 80° C.for 4 days is particularly preferred. This results in a sol having aviscosity of preferably 5 to 100 Pas, particularly preferred 20 to 25Pas.

Subsequently said sol can be processed into a gel film in various ways,for example by casting and gelling on a substrate or in a mold, filmdrawing, blowing techniques or forcing through a dye, the gelling takingplace by evaporation of the solvent and/or addition of gelling agents(e.g. H₂ O, HCl or NH₃). Particularly preferred are casting or doctorblade coating onto a non-adhesive base made of, e.g., polystyrene,polyethylene or teflon and evaporation of the solvent. The gelling ofthe sol may also take place continuously, e.g., on a roller or aconveyer belt.

In a particularly preferred system composed of 90% by moles of MTEOS and10% by moles of TEOS it has surprisingly been found that with basiccatalysis and a temperature of 80° C. said system does not condense toany substantial degree even after a period of 100 h. This is incontradiction to conventional experience according to which the presenceof basic catalysts accelerates the condensation reaction. On the otherhand, the system according to the present invention becomes almostcompletely hydrolyzed but not condensed, so that no gelling takes place.As a result thereof, upon pouring the sol into a mold, the solvent mayevaporate without gelling taking place. Only after the solvent hasevaporated almost completely, a gelling process with formation of a gelfilm sets in.

The resulting gel film is then dried, preferably at temperatures of fromroom temperature to 100° C. and under normal pressure or reducedpressed. Particularly preferred drying conditions are 20 to 30° C. for 3h, followed by 15 h at 50° C.

The gel body obtained can optionally be subjected to variouspretreatments. For example, it can be shaped by mechanical or chemicaltreatments, e.g., drilling, cutting, preliminary dissolution, etching,structuring (embossing, bombardment with ions), folding or bending.

In order to improve the mechanical properties, the gel film mayoptionally be placed into functional (reactive) solvents or be treatedtherewith, e.g., with water, alcohols, amines, Si compounds (e.g. TEOS),or may be treated with reactive gases such as HCl or NH₃.

Optionally a pretreatment of the gel film by means of corona or plasmagenerators may also be carried out.

Finally, a heat-treatment of the gel film within the temperature rangeof from 100 to 1400° C. is carried out, an annealing taking place below200° C., the organic components being burnt out in the range of from 200to 700° C. and a thermal densification (sintering) taking place above700° C.

The heat-treament can be effected by, e.g., heating, irradiation withinfrared, laser or flash-light radiation (Rapid Thermal Annealing).Heating rates of preferably 40 to 50° C./h are employed in saidtreatments.

The heat-treatment can be carried out in various gas atmospheres, e.g.,in air, oxygen, nitrogen, ammonia, chlorine, carbon tetrachloride orcorresponding gas mixtures.

The thermal densification in air is suitable for preferably thinner SiO₂films having a thickness of up to 50 μm. In that case thin transparentglass films are formed at 1000° C. Sintering in nitrogen at 1000 to1250° C. results in black glass films having occluded carbon particlesand showing high strength and being suitable, e.g., as substrates or forlaminates. By sintering in ammonia it is surprisingly possible toproduce also thicker glass films of excellent transparency.

The SiO₂ films according to the present invention show no tendency fortear formation, undesired peeling off or rolling up. Additionally, ithas surprisingly been found that the organic components employed can beremoved thermally without any problems and without any formation oftears so that colorless and transparent glass films whose dimensions aresolely limited by the size of the base may be produced.

The SiO₂ films according to the present invention are suitable for themost diverse applications, e.g., as membranes, filters, components oflaminates or support materials having functional additives such asmagnetic particles, carbon, metal colloids, dyes (including photochromicones) and pigments incorporated therein. Further fields of applicationare optical and electronic components as well as multi-layer materialsfor bullet-proof glazing.

THE FOLLOWING EXAMPLES ILLUSTRATE THE PRESENT INVENTION. EXAMPLE 1

A mixture of 0.92 moles of MTEOS, 0.08 moles of TEOS and 0.25 moles ofcolloidal SiO₂ (MA-ST from Nissan Chemicals; 30% in methanol) is dilutedwith 4.5 moles of ethanol (absolute) and hydrolyzed with 4 moles ofaqueous ammonia (0.34 g of 25% aqueous ammonia in 73 g of water) withmagnetic stirring.

Stirring is continued for another 3 minutes. Upon aging the reactionmixture at 80° C. in a closed vessel for 4 days a viscous sol having aviscosity of 20-25 Pas is obtained.

Said sol is directly cast or doctor blade coated onto polystyrene molds,drawing speeds of from 5 to 20 mm/s being employed. Then the polystyrenemold is covered and kept in an oven at temperatures of from 40 to 65° C.for 15 h. During said period of time the visous sol gels and a gel filmfree of tears and distortion can be removed, said film being dried at20-30° C. for 3 h and subsequently at 50° C. for 15 h.

The gel films produced in said manner may be sintered in air attemperatures of up to 400° C. regardless of their thickness, whichresults in transparent glass films. Gel films having a thickness below50 μm may be sintered in air at up to 1000° C. without tear formation toform transparent glass films.

In the case of sintering thicker films in nitrogen in the range of from400 to 1250° C. mechanically and thermally stable black glass films areformed, which films do not show any visible change upon subsequentlyheating to 1300° C. The thermo-mechanical properties thereofapproximately correspond to those of pure silica glass.

In the case of sintering in an atmosphere of ammonia at temperatures ofup to 1000° C., tearfree glass films of excellent transparency may beobtained also with thicker gel films. The SiO₂ films sintered in ammoniaexhibit better chemical and thermal stabilities and higher breakingstrengths, microhardness and higher temperature-resistance than puresilica glass.

EXAMPLE 2

In order to prepare photochromic glass films a 1×10⁻³ solution of aphotochromic spiropyran dye in methanol is prepared and 1 ml of saidsolution is added dropwise to 10 ml of the sol prepared as in Example 1and showing a solids content of almost 30% by weight. The furtherprocessing is carried out as described in Example 1.

EXAMPLE 3

In order to prepare magnetic glass films, 2.5 ml of a suspension ofnanoscaled maghemite particles having a primary particle size of 10 nmand a solids content of 6% by weight are added dropwise to the solprepared as in Example 1. The further processing is carried out asdescribed in Example 1, the sintering in air resulting in transparent,reddish brown colored magnetic glass films.

We claim:
 1. A process for the production of a thin SiO₂ film whichcomprises:hydrolysis and condensation of (a) 40 to 100% by weight of oneor more silanes of general formula (I)

    R.sub.x --Si--A.sub.4-x                                    (I)

whereinthe radicals A are the same or different and represent hydroxylgroups or hydrolytically removable groups, the radicals R are the sameor different and represent hydrolytically non-removable groups, and x is0, 1, 2, or 3, provided that x≧1 in at least 70% by moles of saidsilanes; optionally in the presence of (b) 0 to 50% by weight ofcolloidal SiO₂ and/or (c) 0 to 10% by weight of an organic binder;processing the resulting viscous sol into a gel film, and heat-treatmentof said gel film.
 2. A process according to claim 1 wherein saidhydrolysis and condensation are carried out in the presence of an acidicor basic condensation catalyst.
 3. A process according to claim 1wherein the hydrolyzed and condensed product is converted into a viscoussol by tempering at temperatures of from 40 to 120° C. for several hoursto several days.
 4. A process according to claim 1 wherein said sol isprocessed into a gel film on a non-adhesive base.
 5. A process accordingto claim 1 wherein said gel film is dried and heat-treated attemperatures of up to 1400° C.
 6. A process according to claim 1 whereinthe silanes comprise 90% by moles methyltriethoxysilane and 10% by molestetraethoxysilane.
 7. A process according to claim 1 wherein colloidalSiO₂ is present.
 8. A thin SiO₂ film produced by a process wherein:(a)40 to 100% by weight of one or more silanes of general formula (I)

    R.sub.x --Si--A.sub.4-x                                    (I)

whereinthe radicals A are the same or different and represent hydroxylgroups or hydrolytically removable groups, the radicals R are the sameor different and represent hydrolytically non-removable groups, and x is0, 1, 2, or 3, provided that x≧1 in at least 70% by moles of saidsilanes; optionally in the presence of (b) 0 to 50% by weight ofcolloidal SiO₂ and/or (c) 0 to 10% by weight of an organic binder arehydrolyzed and condensed, the resulting viscous sol is processed into agel film, and said gel film is heat-treated.
 9. A thin SiO₂ filmaccording to claim 8 having a thickness of from 2.5 to 250 μm and asurface area of at least 25 cm².
 10. A membrane, filter, component of alaminate, or support material having functional additives incorporatedtherein, comprising a thin SiO₂ film according to claim
 9. 11. A thinSiO₂ film according to claim 9 wherein the silanes comprise 90% by molesmethyltriethoxysilane and 10% by moles tetraethoxysilane.
 12. A thinSiO₂ film according to claim 9 wherein colloidal SiO₂ is present.
 13. Athin SiO₂ film according to claim 9 wherein said sol is processed into agel film on a non-adhesive base.
 14. A membrane, filter, component of alaminate, or support material having functional additives incorporatedtherein, comprising a thin SiO₂ film according to claim
 8. 15. A thinSiO₂ film according to claim 8 wherein the silanes comprise 90% by molesmethyltriethoxysilane and 10% by moles tetraethoxysilane.
 16. A thinSiO₂ film according to claim 8 wherein colloidal SiO₂ is present.
 17. Athin SiO₂ film according to claim 8 wherein said sol is processed into agel film on a non-adhesive base.